The antimicrobial effect of various metals and their salts has been known for centuries. Hippocrates wrote that silver had beneficial healing and antidisease properties, and the Phoenicians stored water, wine, and vinegar in silver bottles to prevent spoiling. In the early 20th century, silver coins were put in milk bottles to prolong the milk's freshness. Its germicidal effects increased its value in utensils and as jewelry. The exact process of silver's germicidal effect is still not entirely understood, although theories exist. One of these is the “oligodynamic effect,” which qualitatively explains the effect on some microorganisms, but cannot explain antiviral effects. Silver is widely used in topical gels and impregnated into bandages because of its wide-spectrum antimicrobial activity.
The oligodynamic effect is demonstrated by other metals, specifically gold, silver, copper, zinc, and bismuth. Copper is one such metal. Copper has long been used as a biostatic surface to line the bottoms of ships to protect against barnacles and mussels. It was originally used in pure form, but has since been superseded by brass and other alloys due to their lower cost and higher durability. Bacteria will not grow on a copper surface because it is biostatic. Copper alloys have become important netting materials in the aquaculture industry for the fact that they are antimicrobial and prevent biofouling and have strong structural and corrosion-resistant properties in marine environments. Organic compounds of copper are useful for preventing fouling of ships' hulls. Copper alloy touch surfaces have recently been investigated as antimicrobial surfaces in hospitals for decreasing transmission of nosocomial infections.
The antimicrobial properties of silver stem from the chemical properties of its ionized form, Ag+, and several mechanisms have been proposed to explain this effect. For example, silver ions form strong molecular bonds with other substances used by bacteria to respire, such as enzymes containing sulfur, nitrogen, and oxygen. When the Ag+ion forms a complex with these biomolecules, they are rendered inactive, depriving them of necessary activity and eventually leading to the bacteria's death. Silver ions can also complex with bacterial DNA, impairing the ability of the microorganisms to reproduce. The mechanism for copper ions, on the other hand, is not so well understood. Numerous scientific investigations have focused on the role of the metal form of copper, and have concluded that multiple mechanisms may be possible for copper's antimicrobial effect, including increased production of reactive oxidation species such as singlet oxygen and hydroxide radicals, covalent binding of copper metal to reactive sites in enzymes and co-factors, interference with lipid bilayer transport proteins, and interaction of copper ions with moieties of microorganisms analogous to what have been proposed for silver ions.
It is clear that silver and its various compounds and salts have been the overwhelming favorite in terms of its use as an antimicrobial agent. However, silver in the form of the silver halides silver iodide, silver bromide and silver chloride is well-known to be light-sensitive and was used for many years in photography. Copper, aside from its use in preserving marine objects such as ship hulls, has not generally been used in antimicrobial compounds.
Provision of the oligodynamic metal species in the form of fine particles, including the form of nanoparticles, avoids problems such as settling of the particles in solutions—but introduces a complication in trying to estimate the solubility for a given small particle size or the concentration of free ions produced by contact of specific aqueous solutions with a given set of nanometal particles, in addition to the ubiquitous issue of agglomeration. Use of oligodynamic metal species in the form of nanoparticles introduces a further observation—viz., based on several reports in the literature, such particles may under some (generally unspecified) conditions be taken up by the outer membranes of pathogens and transported into the bodies of the pathogens. In many cases, it is expected that this observation would be advantageous for the antimicrobial effectiveness of the metal species.
It is presently unknown under what precise conditions does such penetration by specific nanoparticles of oligodynamic materials take place; and it is certainly unknown what conditions (including particle size and chemistry) promote or mitigate against such penetration. What is needed are better broad-spectrum antimicrobial compositions that may better target oligodynamic metal compounds to microbes and other pathogens.